Polyolefins, compositions containing them, and articles manufactured using them are prone to degradation accelerated by oxygen. The negative effects of oxygen are even more severe in the presence of heat or ultraviolet radiation. It is, therefore, customary to add chemicals often referred to as antioxidant stabilizers to polyolefins to inhibit the degradation caused by oxygen.
Vitamin E, also referred to as .alpha.-tocopherol (.alpha.-T), and related chromanols are among the most active antioxidants known due in large part to their exceptional ability to react with alkylperoxy radicals, thereby limiting the chain length of damaging oxidation chain reactions. A study of several chromanol antioxidants revealed that Vitamin E and related chromanols are capable of reacting as much as 370 times more rapidly with an alkylperoxy radical than 2,6-di-tert-butyl-4-methylphenol (BHT).
Despite their high inherent activity, chromanols have not gained general acceptance as antioxidants for polyolefins, due to the lack of permanence of chromanols in polyolefins. Lack of permanence generally is understood to mean that the chromanol can migrate out of the polyolefin, evaporate during storage or compounding of the polyolefin, and/or be extracted from the polyolefin by water or other aggressive media. Antioxidant permanence in polyolefins is made difficult by the high temperatures used to process polyolefins as well as the demanding environmental conditions to which articles containing polyolefins are subjected. Examples of demanding environmental conditions include exposure to rain (outdoor articles), food (food packaging), and hydrocarbon grease (telecommunication cable).
There is an on-going need to provide antioxidant stabilizers which are permanent in a polyolefin and which inhibit oxidation under demanding environmental conditions.